It is well known that a thin metal film can absorb substantial amounts of microwave energy and convert this energy into thermal energy for heating a variety of food products. These thin metal films are commonly called susceptors. The susceptor is typically associated in conductive heat transfer relationship with a food product contained in the package and is usually bonded to a structural supporting sheet. There have been many attempts to provide food packages or composite materials that become hot when exposed to microwave radiation.
Many snack foods and other pre cooked foods are currently packaged in some type of bag or carton which also utilizes metallized film in combination with a material having structural properties. The purpose of the metallized film in this context is to create an oxygen barrier and a water barrier to extend the shelf life of the food product at the retail location. In the past, some packaged foods have utilized a laminate of a thin foil and a polymer film. The foil provides a barrier to ultraviolet radiation, oxygen and water vapor while the film provides strength against punctures and tearing. Recently, however, most of these constructions have been replaced by a film or a laminate thereof which has been metallized with a layer of aluminum. This construction is more cost-effective than the foil laminated structure.
These metallized bags cannot conductively heat food because they have too thick a metallic coating to act like a susceptor. These metallized bags also reflect a substantial amount of incoming microwave energy and they effectively prevent microwaves from heating the food directly. Further, in the case of barrier metal coatings on a polymeric substrate, only a small amount of microwave reflection can be achieved before arcing occurs, destroying the barrier properties of the polymeric sheet as well as the structure of the metallic coating. Arcing may also adversely affect the microwave oven and in some circumstances may even result in a fire. Thus, current packaging structures that contain a foil or metallized film coating to act as an impermeable barrier to prolong shelf life cannot also be used to heat food in a microwave oven without fear of fires or arcing because they have a tendency to reflect up to 100% of the energy away from the food.